freezing of my computer

im not a computer buff, so when answering this please try use simple terms. i recently put in a dvd/ cd drive, and loaded the additional software that came with it. but since then when i play games like alias, sims2 and tomb raider after about 30min of play it freezes, not only the game but the computer too, ctrl, alt, del doesnt work only reseting the computer does. pls help!!!!

thanks for the reply, ok but before i loaded the dvd/cd drive my computer did not freeze. i also have deleted the tomb raider and other games that took up a lot of memory. so is it still the graphics card? which one should i get?
oh and soz bout the exclamations marks only read rules aftawards.